The present invention relates in general to a photonic bandgap fiber, and more particularly, to a laser device operative to emit an optical beam comprising multiple wavelengths.
In conventional optical fiber, total internal reflection is responsible for the guiding of light therein. Based on the principle of total internal reflection (TIR), an optical fiber typically consists of a central core surrounded by a cladding layer whose index of refraction n2 is slightly lower than that n1 of the core. The optical fiber is characterized by a normalized frequency as a function of the radius of the core and the core-cladding index difference, which is itself a function of the wavelengths of the guided optical beam. The normalized frequency determines the number of modes supported by the fiber. The dependence on wavelength indicates that conventional filter can maintain single-mode propagation area over a limited wavelength range. The wavelength range is material dependent, and is typically between 10% and 50% of the central wavelengths. For example, if the central wavelength at which the fiber propagates only single mode is 3 micrometers, the range is typically between 0.3 and 1.5 micrometers, which indicates 2.85 to 3.15 micrometers on one extreme and 2.25 to 3.75 micrometers on the other. Beneath the low end of the wavelength range, propagation of multiple modes is supported. Above the high end of the wavelength range, no modes are supported without very high losses or very stringent restrictions on bending, vibration, and micro-discontinuities in the fiber.
In contrast to conventional fibers, photonic bandgap fibers do not depend on reflection from slight differences in index of refraction of material; but rather, they depend on reflection from “bandgaps” created by microstructures (holes) in the fiber. Such fibers are called “photonic bandgap fiber” (PBG). The wavelength dependence of the modal propagation in photonic bandgap fibers is based on different phenomena and equations than total internal reflection. This renders photonic bandgap fibers to allow “endlessly single-mode” propagation which theoretically supports single-mode propagation over an infinite wavelength range. Practical single-mode propagation is then limited to the transparency range of the photonic bandgap fiber material.
Among various optical non-linear characteristics, Raman scattering is an important nonlinear process that shifts the input optical wavelength to longer wavelength. Such process is called as Stokes shift. In a typical fiber Raman laser, a single-mode fiber is placed inside a Fabry-Perot cavity formed by partially reflecting mirrors. Also, a prism can be installed between the mirrors to allow tuning of the laser wavelength by dispersing spatially various Stokes wavelengths which can be selected by rotating the mirror at the output side of the single-mode fiber. The threshold of the fiber-Raman laser is as large as about 500 W when a short fiber length, for example, 1.9 m is used. To reduce the threshold of the fiber-Raman laser to about 1 W, a fiber as along as 10 m is required. In addition to the length requirement, although the output wavelength is tunable in a broad range, the fiber-Raman laser is only operative to output a single wavelength at one time.
Various approaches, for example, diode-pumped solid state lasers frequency-shifted by optical parametric oscillators, and semiconductor (optically- or electrically-pumped heterojunction and quantum well) lasers, have been made to realize a simultaneous multiple-wavelength output, but have various drawbacks such as limited wavelength range, fixed wavelength and large divergence angle instead of near-diffraction-limited, which is typically the convergence of a beam required in many system. The optical parametric oscillator laser includes an optically nonlinear crystal to convert a pumped laser wavelength into a signal wavelength and an idler wavelength. The output of the semiconductor diode layers is typically less monochromatic (wider spectral output range) than other types of lasers; however, because of the small aperture size, the output beam divergence angle is as much as 20°.
As discussed above, the conventional fiber lasers require relatively long fibers to lower the threshold, and the output is monochromatic rather than multiple wavelengths. The heterojunction and quantum well lasers, though output multiple wavelengths, have beam divergence as much as 20°. Therefore, to obtain a near-diffraction-limited optical beam of multiple simultaneous, coaxial wavelengths with a compact volume inexpensively, a new design of laser system is required.